In an apparatus for chemical vapor deposition (CVD), a reactive gas is introduced into a vacuum reaction chamber, flows through a showerhead, and reaches a susceptor or a substrate holder on which a substrate is located. The reactive gas causes chemical reaction on the substrate to form a desired film. As a means to provide energy necessary to induce chemical reactions on the substrate, a method of simply heating the substrate or atomically exciting the reactive gas, such as making plasma, is widely used. After the reaction is finished, byproduct gases are removed from the reaction chamber by an exhaust system including a vacuum pump, then, passing through a purifying system, finally, being discharged into the atmosphere. However, since it is very important to prevent undesired particle deposition on a wall of the reaction chamber or the showerhead during a deposition process, it is preferable that the reactive gases do not react with each other in a gaseous state. Unfortunately, if reactive gases whose decomposition temperatures are substantially lower than 200° C. like metal-organic compounds are mixed in the reaction chamber, the mixture may cause homogeneous reactions in the gas phase leading to a generation of contaminant particles, or cause heterogeneous reactions on a solid-state surface such as a showerhead surface or a reaction chamber wall. Particularly, it may happen that the reactive gas is sensitive to a specific material. For example, zirconium tert-butoxide (Zr(OC4H9)4) is extremely sensitive to moisture, which is strongly like to form zirconium hydroxide (Zr(OH)x) of white powder type. The moisture could have been physically adsorbed on the inner side of the reaction chamber, but it may be also generated over the substrates as a byproduct gas. Then, the moisture reacts with Zr(OC4H9)4 on the inner wall of the reaction chamber or the surface of the showerhead, depositing zirconium hydroxides. The unwanted deposits are eventually flaked off into fine particles due to a repeated thermal expansion and contraction and/or a lattice parameter mismatch between the surface materials and the deposits. As a result of this, the film formed on the substrate may be contaminated and the productivity becomes deteriorated due to a shortened preventative maintenance cycle time to remove the unwanted deposits.
When a highly integrated semiconductor is manufactured, contaminant particles may cause a pattern defect such as a short or disconnection between lines and the size of the contaminant particle influencing yield is in proportion to the line width. Therefore, as the line size becomes smaller, that is, as the density of the integration is increased, the size of particle influencing yield becomes smaller, whereby the number of contaminant particles to be permitted in the reaction chamber is more seriously limited.
FIG. 1 is a brief sectional view of a showerhead of a prior art, U.S. Pat. No. 6,626,998, wherein a plural number of reactive gases flow through without being mixed and are injected over a substrate so as to prevent reactions between the reactive gases therein. As each reactive gas is supplied to the first ring type individual channels 23 through a plurality of gas supply passages 17, the gases are diffused in the first individual channels 23, and then, transmitted to the second ring type individual channels 27 through a plurality of transition passages 25 formed at the bottom of each channel. After being diffused in the second channels 27, the reactive gases are supplied over a substrate through a plurality of second gas transition passages 31 which are formed at the bottom of the second channels. The reactive gases cause chemical reaction on the substrate (not shown) placed on a susceptor keeping temperature of the substrate higher than that of surroundings to form a desired film on the substrate.
FIG. 2 is a brief sectional view of a prior art showerhead as described in JP2005-129712. The 1st purge gas injection holes 10b surround the reactive gas injection holes 10a and the 2nd purge gas injection holes 10 care arranged by proper intervals between the 1st purge gas injection holes 10b. In this configuration unwanted film deposition at the bottom of the showerhead would be suppressed by the work of the 1st and 2nd purge gases used.